Just announced today in Science, researchers at the Marshall University and the Los Angeles County Museum described the presence of fossil young inside the body of the plesiosaur: Polycotylus latippinus. The results of their find seem to confirm what has been suspected for quite some time now, that plesiosaurs were viviparous animals.
O’Keefe, F.R., Chiappe, L.M. 2011. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science. Vol.333(6044):870-873
The evidence had been mounting for some time now. While plesiosaurs came in numerous shapes and sizes, most of those sizes were in the large to giant range measuring in at multiple tonnes (e.g. Liopleurodon and Kronosaurus). That is a lot of weight to attempt to drag up on a beach for egg laying. Further, though the rib cage is well braced ventrally, the limb girdles are not braced against the vertebral column. This would make it very hard for a large landlubbing plesiosaur to make any kind of headway as the limbs would have no leverage against the body for dragging itself on land.
Lastly, and perhaps most importantly, we have known of at least one plesiosaur fossil that had embryos in it. This has been known for at least five years now (I learned of it four years ago, and it has been hinted at before [Smith 2008]). Sadly this specimen still remains unpublished. This new paper by O’Keefe and Chiappe goes on to mention the relatively large size of the young, estimated at 1.5 meters when born. This was much larger than the young of other large extinct and extant marine reptiles. The authors (cautiously) suggest that this might hint at a different life history for plesiosaurs vs. other marine reptiles. They posit that plesiosaurs might have nurtured a small amount of relatively large young, which in turn might have meant that they were more social than previously thought.
Naturally this has resulted in the inevitable comparison to whales. While a “pod of plesiosaurs” does sound interesting, we have far too little evidence to say if such a thing ever happened (and the authors state this too). What we do know is that young plesiosaurs have been found in shallow marine settings. These have been posited to have been “nurseries” where young could stay out of sight from predators while reaching adult size (Martin et al. 2007). Whether, or not adults stayed around, or if they joined a “pod” later (if at all) is all unknown. Still, it is nice to see some validation to what seemed almost necessary for so long.
Admittedly not everyone is convinced (a good thing to see in science). Dr. Ken Carpenter of the Utah State Museum offered Science magazine a dissenting view, suggesting that the position of the young could still indicate that these were juveniles that had been eaten. The O’Keefe and Chiappe considered this in the paper and pointed out that the skeletons lacked any signs of acid etching, as well as showed numerous skeletal bones that did not appear fully ossified. Further analysis could shed more light on this. Publishing on that other plesiosaur could really help things out too.
Assuming that we are looking at viviparous plesiosaurs, that just leaves two other large marine reptile groups of the Mesozoic. Turtles and Crocodylomorphs. In both cases we have extant animals that are obligate oviparous animals, but there might still be reason to think that live birth might have evolved in these groups too. Again, much like with the plesiosaurs, the groups in question (protostegid sea turtles and the podocnemid Stupdendemys, as well as metriorhynchid crocodylomorphs) have members that grew extremely large. While Protostega gigas may have been able to haul itself out on land as extant leatherbacks (Dermochelys coriacea) do, it seems harder to justify that in the much larger Archelon ischyros; an animal that has been estimated to tip the scales at 2 tonnes. Given the amount of effort it takes a large female leatherback (~1 tonne) to haul herself up and down a beach (not to mention the damage it causes to the animals in the short term), it would be all the more amazing if A.ischyros was able to pull off such a feat. The same would go for the metriorhynchids, who had adapted completely to a marine lifestyle (i.e. they had flippers and a tailfin). If a 5 meter Gavialis gangeticus can barely move around on land, I’d hate to see what a 5 meter Dakosaurus would look like. To date we have no evidence one way, or the other for these last two groups. There is a bit more resistance to the idea of viviparity in these groups as no extant members exhibit viviparity. This has lead some to wonder if the calcified eggs of archosaurs (and many chelonians) might prove a phylogenetic constraint on live bearing (the young absorb calcium from the shell, which could mess up calcium absorption in a taxon evolving along the lines of viviparity). The chelonian shell — in turn — may also have been constraining on the size of young that can be held in the body cavity. Still, to date, there are no nests, eggs, or embryos for any of these taxa, thus leaving the matter open for debate. It is interesting that neither protostegids, nor metriorhynchids got to the huge sizes of mosasaurs, ichthyosaurs and plesiosaurs, but that could have been for any number of reasons including the simple lack of finding the larger taxa yet. Until then the physics vs. phylogeny argument remains unresolved.
Anyway, compelling evidence for live bearing in at least some plesiosaurs. Woohoo!
~Jura
References
Martin, J., Sawyer, F., Reguero, M. Case, J.A. 2007. Occurrence of a Young Elasmosaurid Plesiosaur Skeleton from the Late Cretaceous (Maastrichtian) of Antarctica. 10th Int.Symp.Antarctic Earth Sciences.
O’Keefe, F.R., Chiappe, L.M. 2011. Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia). Science. Vol.333(6044):870-873
Smith, A.S. 2008. Fossils Explained 54: Plesiosaurs. Geol.Today. Vol.24(2):71-75
